1. Field of the Invention
The present invention relates to a lubrication device for a dry-sump, four-stroke engine formed with through holes for absorbing a fluctuating pressure within a crank chamber, which is installed for example in a wall separating a clutch chamber and the crank chamber, and relates in particular to a mechanism for accelerating the return of lubricant oil to the crank chamber during a piston's movement from a bottom dead center position to a top dead center position.
2. Description of Related Art
In dry-sump, four-cycle engines not possessing an effective oil pan in the bottom of the crank chamber, when the diameter of a crank web is enlarged to increase the inertial mass of a crankshaft, the outer circumferential surface of the crank web nears the bottom of the crank chamber.
The increase in pressure inside the crank chamber while the piston falls downward from the top dead center to the bottom dead center is especially large in large displacement single-cylinder and V-type two-cylinder engines so that the lubricant oil in the vicinity of the crank web is blown away due to the air pressure that accompanies the fall of the piston.
Motorcycle engines, however, contain a clutch chamber housing a wet-type clutch on the side of the crank chamber. This clutch chamber adjoins the crank chamber with the sidewall of the crankcase interposed in between them. In conventional engines, multiple through holes (breather holes) are formed in the sidewall of the crankcase to connect the crank chamber and the clutch chamber in order to alleviate pressure fluctuations within the crank chamber during a downward movement of the piston. These through holes are formed at a position higher than the fluid surface of the lubricant oil within the clutch chamber. When the piston moves downward, these through holes allow gas inside the crank chamber to escape into the clutch chamber.
However when these through holes in the sidewall are open, the lubricant oil is blown away from the periphery of the crank web, and cannot be prevented from flowing into the clutch chamber from the through holes. In view of this problem, oil return holes are formed in the lower section of the crank case sidewall, and the pressure fluctuation in the crank chamber occurring when the piston moves from a bottom dead center position to a top dead center position is utilized to draw into the crank chamber, the lubricant that flowed into the clutch chamber.
The crankcase through holes are always open in both the crank chamber and the clutch chamber. Therefore when a negative pressure occurs in the crank chamber from the piston moving upward, the through holes function as intake holes and a negative pressure acts on the clutch chamber. The pressure differential between the crank chamber and the clutch chamber therefore can be relieved, and this makes it difficult for lubricant oil to return to the crank chamber from the clutch chamber.
Consequently, an increase in the amount of oil, not contributing to engine lubrication, accumulates in the clutch chamber while the engine is running. Lubricant oil has to be refilled by an amount equal to this oil accumulated in the clutch chamber. This situation leads to problems since the oil tank has to be enlarged, the level of lubricant oil inside the clutch chamber rises, and the lubricant oil agitation resistance increases due to the wet-type clutch.
A countermeasure for this problem known in the prior art is the use of dry-sump, four-cycle engines provided with a reed valve in the oil drain port open on the bottom of the crank chamber. This reed valve is designed to open when the pressure in the crank chamber increases and allows the lubricant oil to flow from the crank chamber towards the transmission chamber. In other words, the reed valve closes at the point in time that the piston rises and creates a negative pressure in the crank chamber. A pressure differential is in this way maintained between the crank chamber and the transmission chamber.
The conventional four-stroke engine requires a dedicated reed valve for maintaining a pressure differential between the crank chamber and the clutch chamber while the piston is rising. This requires increasing the number of engine parts, requires drastic changes to the crankcase design to be used for already built engines, and therefore leads to higher costs.
A further problem with the above engine of the prior art is that it also requires providing a space for installing the reed valve at the bottom of the crankcase. The bottom of the crankcase therefore protrudes downward in localized sections. The overall height of the engine therefore becomes larger and the merits of the dry-sump, four-stroke engine are lost.
The reed valve further has a body made of a thin metal capable of resilient deformation according to pressure fluctuations within the crank chamber. This reed valve is adjacent to the outer circumferential surface of the crank web rotating at a high speed. This valve body might collapse and be destroyed if it is repeatedly subjected to pressure from the crank chamber. In that case, the valve body might make contact with the crank web and valve body debris might fly into the crank chamber. Therefore this debris might possibly cause damage to the crankshaft bearing or the section coupling the crank pin and crankshaft.
In view of the problems with the prior art, the present invention has an advantage of providing a lubrication device for a dry-sump, four-stroke engine with a simple design, capable of returning lubrication oil to the crank chamber from another chamber and eliminating cost problems.